In the optical lithography technology field, conventionally research and development of the optical lithography technology using such as g-line, i-line, and KrF excimer lasers have been actively made. From the viewpoint of further downsizing and increase of capacity of semiconductor devices in the future, improvement and development of the lithography technologies utilizing such as an electron beam drawing apparatus using an electron beam capable of drawing fine patterns on a sample, and a scanning electron microscope (SEM) capable of observing such a fine pattern with high resolution are expected.
Generally, an apparatus using an electron beam has a focus adjustment function and an astigmatism correction function so as to downsize the spot shape of an electron beam emitted from an electron gun or an electron beam irradiation apparatus and to make the spot shape a perfect circle. The focus adjustment function is a function to downsize the spot shape of an electron beam formed on the surface of a sample as much as possible. The astigmatism correction function is a function to make the spot shape of an electron beam not an ellipse but an perfect circle.
Conventionally, to correct astigmatism, an image based on secondary electrons or backscattered electrons obtained by irradiating an electron beam onto a sample having a fine structure such as an Au or Pt evaporated sample and an etched sample having a hole shape is observed using the SEM function of an electron beam drawing apparatus. Then an adjustment is made by operating the astigmatism correction function so that the observed image can be seen more sharply. However, in this qualitative determination method, it is difficult to evaluate the existence, the direction, and the size of the astigmatism, and the operation of correcting astigmatism is performed manually. Therefore, there are problems that it requires time and reproducibility is poor. Also problematically, the result is likely to be influenced by an individual's learning characteristics throughout the operations. As a result, this method may be used for research and development, but there are many problems when used in an apparatus for production.
To address the problems, for example, as a technique as described in Patent Document 1 called the knife edge method, there is a proposed technique that, after the spot shape of an electron beam is directly measured, the astigmatism is corrected. However, the challenge is to manufacture a high-accuracy knife edge. Also, there is a problem that enough measurement accuracy (reproducibility) cannot be obtained due to, for example, scattered electrons. To address the problem, there is a proposed technique as described in Patent Document 2 that after a reference pattern having a fan-shaped radial pattern is formed (on a reticle in this case), the astigmatism is determined and corrected by detecting the defocusing direction and the defocusing amount of the electron beam image obtained by irradiating electron beams. However, there is no specific description given regarding the concrete procedure and the quantifying method of detecting the size and the direction of the astigmatism (including after the image is formed) except to note that the resolution of the image is observed. Further, there is a problem that it requires cumbersome procedures and time to make the reference pattern in which a fan-like pattern with the width of the pattern being changing continuously is radically and orderly formed.
Further, in a proposed technique described in Patent Document 3, a reference sample on which plural reference patterns each having a lattice structure made of lines and spaces are discretely disposed along a circumference of a circle or concentric circles so that the lattice direction (lines and spaces direction) is made into the radius direction is prepared, and an electron beam is scanned along the circle passing through the reference patterns to detect the amplitude of an electronic signal to determine and correct an out-of-focus state and the astigmatism. However, since it is necessary to dispose plural reference patterns each of the lattice directions different from others on the corresponding prescribed positions along the circumference, the problem is that it is very difficult to manufacture the reference sample. Further, it is also cumbersome to accurately scan an electron beam along a circumference of a circle or concentric circles. Still further, when the scanning radius of an electron beam is large, aberration due to deflection is disadvantageously added.    [Patent Document 1] Japanese Laid-Open Patent Application No. 2006-080201    [Patent Document 2] Japanese Laid-Open Patent Application No. 2004-153245    [Patent Document 3] Japanese Patent No. 3984019